1. Field of the Invention
This invention relates to polymeric compositions which are readily crosslinkable to produce heat resistant and flame resistant products useful in the production of coated wire and cable products as well as film sheet and molded products. More particularly, the invention relates to crosslinkable ethylene-vinyl ester and ethylene-alkyl acrylate copolymers which during processing, i.e., in the uncrosslinked state, exhibit improved scorch resistance and improved processability and, after crosslinking, exhibit improved resistance to degradation by heat aging.
2. Description of the Prior Art
One of the most important uses of fire resistant polymer compositions is for wire and cable insulation. In electrical environments both insulating and fire resistant properties are considered to be necessary. For flame resistance, extrudable compositions available to the wire and cable art were at one time required to contain halogenated polymers, such as chlorinated polyethylene, polyvinyl chloride, chlorinated polybutadiene, chlorinated paraffin, etc., together with antimony trioxide. It was necessary that both components be present in sizable quantities. Alternatively, a coating of chlorosulfonated polyethylene paint was applied to a nonflame retardant insulating compound which required an additional manufacturing operation.
In certain applications a problem existed in that electrical failures occurred due to migration of the organic insulating component used. The problem was solved through the addition of hydrated alumina to compositions whose organic binder consisted of butyl rubber, epoxy resins or polyester resins. Such compositions are disclosed in Kessel et al U.S. Pat. Nos. 2,997,526, 2,997,527, and 2,997,528. These compositions, however, failed to possess an acceptable balance of processability and extrudability characteristics, physical and electrical properties, heat resistance and flame resistance. Furthermore, these compositions exhibited unacceptable tensile strength, elongation and percent elongation after aging.
Fire retarding polymeric compositions exhibiting improved moisture and heat resistance comprised of a crosslinkable polymer, such as ethylene-vinyl acetate copolymer, one or more silanes and one or more hydrated inorganic fillers have found wide acceptance in the wire and cable industry. Such compositions are disclosed in U.S. Pat. Nos. 3,832,326 and 3,922,442 to North et al and U.S. Pat. Nos. 4,349,605 and 4,381,362 to Biggs et al. Besides the crosslinkable polymer, silane and hydrated filler, additives such as pigments, stabilizers, lubricants, and antioxidants are typically incorporated. These formulated compositions exhibit a unique balance of processability, improved physical and electrical properties together with a high degree of flame and fire retardance. Moreover, these highly desirable results are achieved (a) without the use of halogenated polymers, such as polyvinyl chloride and chlorosulfonated polyethylene, thereby eliminating potential for generating dangerous hydrogen chloride fumes; (b) without the use of carbon black thereby making it possible to formulate colored insulations; (c) without the application of any flame retardant coatings thereby eliminating the need for an additional step in manufacturing operations after the insulating compound is extruded onto the conductor; and (d) without the use of antimony trioxide thereby eliminating the need to use a substantial quantity of an expensive compounding ingredient.
The compositions of North et al and Biggs et al find particular use as white and colored insulation compositions which can be extruded over metal, e.g., copper or aluminum, conductors to provide a single layer insulating and jacketing composition which meets the automotive primary SAE J1128 standards and UL 125.degree. C. appliance wire SIS standards. These insulating compositions are particular useful for the insulation of switchboard wire, appliance wire and automotive wire where a unique combination of superior electrical properties combined with resistance to the degradative effects of heat and flame are essential and where low smoke density and non-corrosive fumes are desirable.
Antioxidants disclosed to be useful for the North et al and Biggs et al compositions include polymerized 1,2-dihydro-2,2,4-trimethyl quinoline, distearyl-3,3'-thiodiproponate (DSTDP), and combinations of DSTDP with hindered phenols, such as tetrakis(methylene(3,5-di-t-butyl-4-hydroxyhydrocinnamate)) methane. Lubricants which are disclosed include fatty acid soaps, such as calcium stearate and aluminum stearate, silicone oils, long chain aliphatic amides, natural and synthetic waxes and low molecular weight polyethylenes. A combination of lauric acid and ethylene-bis-stearamide is disclosed to be an especially useful lubricant.
Low molecular weight products which are mixtures of light-colored aliphatic resins having molecular weights below 2000 are used as processing modifiers for plastic compounds. Literature for these processing agents indicates that they possess a natural tackiness at processing temperatures which facilitates uniform blending of highly filled polymer compositions. It is recognized within the industry that if compounding ingredients are not uniformly dispersed, physical properties of the resulting compositions are adversely affected. These processing agents are further indicated to provide some viscosity reduction during processing to improve flow characteristics and are suggested for use with TPO compounds, flame retardant formulations and filled polymeric systems. There is no suggestion, however, that the aliphatic resin processing agents can be utilized in crosslinkable ethylene-vinyl ester copolymer compositions of the type disclosed by North et al and Biggs et al containing a silane and hydrated inorganic filler or that significant advantages in the crosslinked product will result therefrom.
It is an object of this invention to provide improved crosslinkable flame retardant polymeric compositions based on ethylene-vinyl ester copolymers and ethylene-alkyl acrylate copolymers which exhibit superior processing characteristics and have significantly improved resistance to scorch.
It is a further object to provide crosslinked compositions which exhibit improved resistance to oxidative degradation.